Load measurement device

ABSTRACT

A variable reluctance load cell for measuring the static or slowly fluctuating load, or tension, on devices is contained in a support tube. A sensor in the tube utilizes opposing C and I shaped magnetic cores attached to opposing ends of the support tube. A magnetic circuit is formed having an inductance defined by the size of the gap between the magnetic cores with the reluctance dominated by the gap. The sensor inductance is coupled with a fixed, predetermined capacitance in a resonant LC circuit, and the resonant frequency is a function of the gap. The sensor is in a cavity within the tube, and the cavity is sealed in a manner that prevents water or other damaging agents from entering the sensor. In this manner, mounting the sensor and tube to a static device and measuring the AC voltage at the sensor, the amount of load, or stress can be determined.

This is a continuation of Ser. No. 09/842,564 filed on Apr. 26, 2001,now U.S. Pat. No. 6,422,089.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to load cells for measuring static andslowly fluctuating load. More particularly, the present inventionrelates to a variable reluctance load cell for measuring the load, ortension, on static devices in an environmentally hostile environment(e.g., determining mooring line tensions of offshore oil platforms).

2. Description of the Related Art

Offshore deepwater platforms utilize various mooring systems forstationkeeping. A number of offshore platforms e.g. spars, deep draftcaissons, semisubmersibles and floating production, storage andoffloading vessels (FPSOs) are equipped with a means to jack the mooringchain and maintain tension on the line, reducing the amount of slack.Passively moored platforms or vessels that are not required to move maynot be equipped with a permanently mounted tensioning system. As themooring lines experience fatigue and stretch, the platform can twist,leading to increased friction between the links, and acceleratingfatigue and failure. The amount of tension on the mooring linedetermines the amount of slack, and consequently the amount of relativemovement of the platform or vessel.

Fairleads are used to attach the mooring chain to the deck of theplatform or vessel. In one configuration, a chain stopper latches thechain outboard of the fairlead and allows the stopper to rotate freelyabout two perpendicular axis. All motion change between the mooring lineand the vessel occurs on proper bearing surfaces, and not between thefairlead and the chain.

Tension in the vertical chain leg between the fairlead and the decklevel stopper, combined with the rotation of the fairlead caused byyawing of the platform or vessel, promote undesirable wear in the chainlinks.

Similarly, suspension bridges rely on large cables to maintain supportfor the bridge span. The amount of tension on the suspension cables isindicative of the stress placed on the cables, and the amount and rateof cable wear and or fatigue.

Various devices are available to measure the amount of tension, orapplied tensile force, placed on fasteners and securing lines, includingstrain gauge bridges, differential transformers, capacitance sensors andvariable reluctance load cells.

Mooring line tensions have been measured with instrumented chain linksthat employ strain gauged shear pins, strain gauges and strain gaugeload cells. Generally, these devices have not been reliable for longterm applications in hostile environments, for example, marine andaerospace environments. Strain gauges require adhesive attachment to thesurface being measured. In non-controllable environments, strain gaugesare subject to drift caused, for example, by adhesive breakdown,requiring recalibration. In environmentally hostile environments, thefrequency of recalibration, repair or replacements becomes expensive,and may even be dangerous to perform.

Variable reluctance load cells use a variable reluctance transducer tomeasure force. For example, a core and winding can be used to sensechanges in proximity to a cantilevered spring. Changes in inductance,caused by changes in the gap between the core and the spring, arereflected in the frequency of an oscillator circuit. In a previous loadcell sensor utilizing a variable reluctance transducer, the load cellsensor responded primarily to forces along a pre-selected axis, whilebeing relatively insensitive to both forces along axis transverse to thepre-selected axis, and to bending moments. Accordingly, the load cellsensors were mounted in the middle of the structure under observation tocompensate for bending forces. Additionally, the load cell sensor wasinternal to a load-measuring unit, and contained all the components.Intrusion of contaminants into the sensor region could lead to prematureaging of the components, including corrosion, making the readingsunreliable.

Accordingly, there is a need for a load measurement device that is lessprone to the various effects of exposure to hostile environments and cantake into account effects of bending.

SUMMARY OF THE INVENTION

This invention provides a variable reluctance sensor for measuring theload, or tension, on static devices in an environmentally hostileenvironment.

A sensor in accordance with the invention uses opposing magnetic corescontained in a support tube. Each of the magnetic cores is attached toopposing ends of the support tube. Thus, as the support tube expandsalong the tube axis, the ends of the support tube, which areperpendicular to the tube axis, separate. A magnetic circuit is formedhaving an inductance defined by the size of the gap between the magneticcores. Accordingly, when the magnetic cores attached to the tube endsseparate, the size of the gap between the magnetic cores is increased.Thus, when the inductance is altered, the amount of expansion that hasoccurred can be determined. Knowing the elastic characteristics of thesupport tube material, the amount of force applied to the support tubecan be calculated. Similarly, contraction of the support tube results ina change in inductance that is indicative of the amount of stressreduction. Alternatively the support tube can have very little stiffnessrelative to the structure that it is mounted on so that no load passesthrough the support tube and it merely displaces the same amount as thestructure displaces in the region between the attachment points. Thecombination is tested under known loads to provide the calibration.

Preferably, one of the magnetic cores is generally C-shaped, andattached to an end plate by way of a bracket. The end plate may be oneof the tube ends, or another plate that is in turn attached to thesupport tube. The C-shape is preferred for one of the magnetic cores sothat the windings can be placed at the ends of the C-shaped gaps. Theother magnetic core is preferably I-shaped, and is attached to a secondend plate by way of a second bracket. The second end plate, like thefirst end plate, may be the other tube end, or another plate that is inturn attached to the support tube. Thus, a cavity within the supporttube containing the sensor is formed. Preferably, the cavity containingthe sensor is sealed in a manner to prevent water or other damagingagents from entering the cavity and damaging the sensor or its wiring.The cavity can also be filled with a low durometer elastomeric pottingmaterial, silicon oil, or any other suitable material for protection ofthe components from environmental agents such as water. The choice ofthe elastomeric potting material can be selected according to theanticipated environmental exposure of the sensor. For example, a lowout-gassing material may be appropriate if the sensor is used at highaltitude or space while a low compression material may be better if thesensor is used below sea level, such as underwater or underground.

An excitation coil is wound around the poles on one of the magneticcores, and provides electrical connection for an inductance whose valueis variable as a function of the widths of the gaps, and also the axialdistortion of the support tube. In the preferred embodiment, there aretwo excitation coils, each surrounding a separate end of the C-shapedcore. This arrangement minimizes non-linearity of response due tofringing effects. The wires from the two coils are twisted and attachedto cabling that connects them to external circuitry. Thus, when excitedby an external AC voltage, the C-core, the I-core and the gap betweenthe C and I cores form an element of a magnetic circuit. The reluctanceof this element is dominated by the gap because the C and I cores arefabricated from high permeability magnetic materials having very littlereluctance. The sensor inductance is coupled with a fixed, predeterminedcapacitance in a resonant inductance-capacitance (LC) circuit. Theresonant frequency of the LC circuit is a function of the gap betweenthe C-shaped and I-shaped cores. Accordingly, changes in the gapdimension results in a change in oscillation frequency. Since the onlyactive component in the sensor is the number of excitation coils, thesensor is immune to drift.

To measure the load on a static device, for example, a chain that moorsa marine platform, the support tube is fixedly attached to the surfaceof a sensor link, and the sensor link placed as a link in the chain. Thesupport tube can be attached to the surface of the sensor link usingbolts, by welding, or any other suitable attaching means. In order forthe sensor to measure the load on the sensor link, it is preferred thatthe sensor tube material and the sensor link material are compatible,more preferably the same material or material having the same or similarcoefficient of thermal expansion. In the preferred embodiment, thesupport tube and the sensor link are made of steel. When used in marineapplications, it is preferred that a protective coating is applied tothe support tube and the sensor link.

These and other features and advantages of this invention are describedin or are apparent from the following detailed description of thepreferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The preferred embodiments of the present invention will be described indetail, with reference to the following figures, wherein:

FIG. 1 is a top view of a variable reluctance sensor assembly having avariable reluctance sensor in a support tube attached to a sensor link;

FIG. 2 is a close-up top-view of the variable reluctance sensor;

FIG. 3 is a side view of the variable reluctance sensor assembly; and

FIG. 4 is close-up side view of the variable reluctance sensor.

Throughout the drawing figures, like reference numerals will beunderstood to refer to like parts and components.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 is a top view of a variable reluctance sensor assembly 10 havinga variable reluctance sensor 12 in a support tube 14 attached to asensor link 16. FIG. 2 is a close-up top-view of the variable reluctancesensor 12. FIG. 3 is a side view of the variable reluctance sensorassembly 10, and FIG. 4 is a close-up side view of the variablereluctance sensor 12.

The sensor 12 in accordance with the present invention uses an opposingC-shaped magnetically permeable core 18 and an I-shaped magneticallypermeable core 20 contained in the support tube 14. This forms amagnetic circuit having an inductance defined by the size of the gaps22, 24 between the magnetic cores 18, 20.

The C-shaped core 18 is mounted on a first mounting plate 26 by a firstbracket 28. Likewise, the I-shaped core is mounted on a second mountingplate 30 by a second bracket 32. The first mounting plate 26 ispreferably positioned transverse to the longitudinal axis of the supporttube 14. Similarly, the second mounting plate 30 is also preferablypositioned transverse to the longitudinal axis of the support tube 14.As the support tube 14 expands along its' longitudinal axis, the firstand second end plates 26, 30 separate. Since the C-shaped core 18 andthe I-shaped core 20 are attached to their respective mounting plates26, 30, the size of the gap 22, 24 will be lengthened. This alters theinductance between the C-shaped core 18 and the I-shaped core 20, and isindicative of the amount of expansion that has occurred, and the forceneeded to cause the expansion. Likewise, as the support tube 14contracts along its' longitudinal axis, the first and second end plates26, 30 move towards each other, resulting in the shortening of the gap22, 24. This alters the inductance between the C-shaped core 18 and theI-shaped core 20, and is indicative of the reduction in the forceapplied to the sensor link 16.

Mounted around each end of the C-shaped core 18 is a first excitationcoil 34 and a second excitation coil 36. The two excitation coils 34, 36are connected in series at coil wires 38, and subsequently attached tocabling 40. The excitation coils 34, 36 provide electrical connectionfor an inductance whose value is variable as a function of the widths ofthe gaps 22, 24, and also the axial distortion of the support tube 14.This arrangement minimizes non-linearity of response due to fringingeffects. The coils 34, 36 are preferably encapsulated in anon-conductive material, for example, polyurethane.

When excited by an external AC voltage, the C-core 18, the I-core 20 andthe gaps 22, 24 between the C and I cores 18, 20 form an element of amagnetic circuit. The reluctance of this element is dominated by thegaps 22, 24 because the C and I cores are fabricated from highpermeability magnetic materials having very little reluctance. Thesensor inductance is coupled with a fixed, predetermined capacitance ina resonant inductance-capacitance (LC) circuit, not shown. Manydifferent LC circuits are known in the art, and the actual layout canvary. Accordingly, LC circuit design need not be discussed further. Theresonant frequency of the LC circuit is a function of the gap betweenthe C-shaped and I-shaped cores. Accordingly, changes in the gapdimension results in a change in oscillation frequency. Since the onlyactive component in the sensor is the number of excitation coils, thesensor is immune to drift.

Cabling 40 can be any electrically conductive wires, and is preferablylow capacitance twisted pair whose dielectric constant varies littlewith temperature. Cabling 40 connects the sensor 12 to a connector 42.The connector 42 can be any suitable electrical connector. In thisinvention, when used near or under water, connector 42 is preferably anunderwater mateable connector. The connector 42 allows the cabling 40 tobe connected to external electrical circuitry that supplies electricityas well as connecting the sensor to a resonant LC circuit, not shown.

In the preferred embodiment, mounting plates 26, 30, in conjunction withthe support tube 14, forms a sensor cavity 44. The sensor 12 is enclosedin a protective cavity that can be sealed, to provide additionalprotection against water, marine growth or other agents from damagingthe sensor 12 and its cabling 40. In the preferred embodiment, thesensor cavity 44 is filled with a low durometer elastomeric pottingmaterial that encapsulates the sensor 12 and its wiring. The pottingmaterial can be any of a number of known potting materials, for example,solithanes and room temperature vulcanizing silicones, commonly referredto as RTV. Alternatively, the sensor cavity 44 can be filled with anoil, preferably a silicon oil. Additionally, the external surface 46 ofthe sensor cavity 44 can be covered with an elastomeric boot and shroud,not shown, forming a rugged sheath around the sensor cavity 44 toprotect the sensor 12 and its components from damage caused by droppedor sharp objects impacting the support tube 14 or the sensor cavity 44.

The support tube 14 contains the sensor 12, and is used to mount thesensor 12 to the sensor link 16. Having a support tube 14 longer thanthe sensor 14 increases the gauge length of the sensor assembly 10,providing additional displacement and increasing the accuracy of thedevice. The connector 42 is mounted on the support tube 14, and allowsfor connection of the sensor 12 to the external LC circuit.

The support tube 14 is fixedly attached to the surface 48 of a sensorlink 16. The support tube 14 can be attached to the surface 48 of thesensor link 16 using bolts, by welding, or any other suitable attachingmeans. In order for the sensor 12 to accurately measure the load on thesensor link 16, it is preferred that the support tube material and thesensor link material are compatible. More preferably the support tubematerial and the sensor link material should be the same material ormaterial having the same or similar coefficient of thermal expansion. Inthe preferred embodiment, the support tube 14 and the sensor link 16 aremade of steel. When used in marine applications, it is preferred that aprotective coating is applied to the support tube and the sensor link.

The primary purpose of the support tube 14 is to increase the gaugelength of the assembly and thereby provide additional displacement toincrease the accuracy of the device.

When assembled, the variable reluctance sensor assembly 10 can measuresteady state and slowly fluctuating loads in the range from DC to 10 Hz.This device is capable of measuring displacements to as low as 0.0001inches. Multiplying the displacement by the stiffness of the link yieldsthe load measurement capability. The amount of measured displacement isdependent upon the area of the poles and the opposing area of theI-core, and can range from 10⁻⁴ and larger, including inches and feet.

The device is particularly appropriate for measuring loads on deployedstructures that require long term monitoring and do not provide accessto the sensor for maintenance or replacement. Applications includebuilding monitoring, bridge monitoring, tower monitoring, marinemoorings monitoring, and weighing hoppers of a vehicle. Marine mooringsmonitoring includes large marine platforms such as spars, or a buoy thatfloats vertically in the water and moored to the seabed by anchors, aswell as floating production, storage and offloading vessels andsemi-submersibles and submersibles.

Another embodiment is to mount at least 3 sensors away from the axis ina single housing or in multiple housings and to use the displacementsmeasured by these sensors to define a plane whose equation specifiesboth the displacement along the longitudinal axis and the rotation thusseparately defining the axial tension or compression and the bendingload.

While the invention has been specifically described in connection withcertain specific embodiments thereof, it is to be understood that thisis by way of illustration and not of limitation, and the scope of theappended claims should be construed as broadly as the prior art willpermit.

What is claimed is:
 1. A device for measuring change in load applied toa structure comprising: means for mounting said device to saidstructure; a first magnetically permeable core; a second magneticallypermeable core; means, internal to said device, for supporting saidfirst magnetically permeable core and said second magnetically permeablecore in a manner and an orientation to both provide a gap therebetweenand farm a magnetic circuit, said gap lying in a plane transverse todirection of said load, size of said gap being a function of magnitudeof said load; an electrically conductive wire wound around at least saidfirst core; a capacitance; a capacitance inductance circuit formed fromsaid capacitance being operatively coupled to said electricallyconductive wire, said capacitance inductance circuit having a resonantfrequency; and, means for measuring change in said resonant frequency asa function of change in said gap size and for calibrating said resonantfrequency change with change in said magnitude of said load; wherebysaid change in said load applied to said structure is measured.
 2. Thedevice of claim 1 further comprising a source of electrical excitationlocated external to said cell and operatively coupled to said conductivewire.
 3. The device of claim 1 for use in a hostile environment andwherein said supporting means includes a cavity filled with materialthat protects at least said first core, said second core, and saidconductive wire from damage otherwise resulting from said hostileenvironment.